The Mechanism of the Selective NOx Sorption on H3PW12O40·6H2O (HPW)

NO x absorption/desorption capacities of 12-tungstophosphoric acid hexa-hydrate were measured under representative exhaust gas mixture conditions. The amounts of NO x absorbed and then desorbed are high and equal to 46 of NO2 mgg−1 of HPW. The mechanism of absorption proceeds by substitution of lattice water molecules with formation of a [H+(NO2−,NO+)] complex. During the cooling phase and in the presence of water, around 100°C, reverse substitution occurs. Two possibilities to wash-coat HPW on a monolith are presented. The first one consists in a partial substitution of H+ by a monovalent cation while the second one consists of supporting HPW on a high surface oxide. The anchorage quality is related to the Bronsted acidity, the best candidates for the role of support are SnO2 and TiO2.